Golf ball

ABSTRACT

To provide a golf ball that ensures both an improved durability and an extended flight distance, in which cracks or breakage of a core can be prevented while making the hardness of the core surface greater so as to obtain the greater difference in hardness between the core surface and the core center. The golf ball of the present invention includes a core and a cover that surrounds the core, in which the core includes a base rubber and an unsaturated carboxylic acid or its salt as a co-crosslinking agent, the core has a surface hardness of 80 or more in terms of JIS-C hardness, and the unsaturated carboxylic acid or its salt contained in a region ranging from a surface of the core toward its center by less than 2 mm has an average particle diameter of less than 90 μm.

CROSS-REFERENCE TO RELATED APPLICATION

This Application claims priority from Japanese Patent Application No. 2016-256210 filed Dec. 28, 2016, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball, and more particularly, relates to a golf ball that ensures both improved durability and extended flight distance.

The core of a golf ball is manufactured by molding a rubber composition, in which an unsaturated carboxylic acid or its salt as a co-crosslinking agent is contained in a base rubber such as butadiene rubber, under heat and pressure. In this rubber composition, the unsaturated carboxylic acid or its salt is contained in a large amount with respect to the base rubber. However, the unsaturated carboxylic acid or its salt is more likely to adhere to the surface of the inner wall of a kneading machine during kneading, thereby hindering kneading operation. Furthermore, since the unsaturated carboxylic acid or its salt is not well dispersed in the base rubber, the golf balls may not attain desired performance due to non-uniform hardness, rebound property, or durability.

JP H9-235413 A describes a golf ball having appropriate hardness and showing excellent rebound performance. It describes that since the golf ball has a rubber composition in which 30 to 50 parts by weight of an unsaturated metal carboxylate having an average particle size of 5.0 μm or less and 10 to 30 parts by weight of barium sulfate are contained per 100 parts by weight of base rubber containing polybutadiene rubber at least 40 percent of which has a cis-1,4-structure, workability during kneading and dispersion property in the base rubber can be significantly improved so as to obtain such a golf ball.

JP H11-57069 A describes a golf ball having little unevenness in compression and hardness, and exhibiting an excellent rebound performance and an improved durability by using zinc acrylate having a particle size distribution of 0.1 to 5 μm and an average particle size of 1 to 4.5 μm as a co-crosslinking agent of a rubber composition, so that the reactivity of zinc acrylate during crosslinking can be made uniform.

JP 2003-507206 A discloses a method of manufacturing a golf ball having a multi-layer core.

SUMMARY OF INVENTION

Regarding the hardness of a core formed from a rubber composition, it is known that the hardness of the core surface is greater than that of the core center as is obvious from comparison of them. It is considered that as the difference in hardness between the core surface and the core center increases, the spin rate of a golf ball at the time of impact decreases, thereby increasing a flight distance. However, a golf ball, which adopts a core having a harder core surface to obtain the greater difference in hardness between the core surface and core center, has such problems in durability that cracks occur in the core and the core breaks at the time of impact of the golf ball.

It is therefore an object of the present invention to provide a golf ball that ensures both an improved durability and an increased flight distance, in which cracks or breakage of a core can be prevented while making the hardness of the core surface greater so as to obtain the larger difference in hardness between the core surface and the core center.

To achieve the above-described object, the golf ball of the present invention comprises a core that includes a base rubber, and an unsaturated carboxylic acid or its salt as a co-crosslinking agent, in which the core has a surface hardness of 80 or more in terms of JIS-C hardness, and the unsaturated carboxylic acid or its salt contained in a region ranging from the core surface toward the core center by less than 2 mm has an average particle diameter of less than 90 μm. The average particle diameter represents a value measured by a particle size distribution measuring device using a sedimentation method.

The core may comprise a multi-layer structure having at least a center core located in the center of the core and an outer core that surrounds the center core and has a thickness of 2 mm or more. In this case, the unsaturated carboxylic acid or its salt contained in the center core has an average particle diameter of 90 μm or more, and the unsaturated carboxylic acid or its salt contained in the outer core has an average particle diameter of less than 90 μm.

The unsaturated carboxylic acid or its salt contained in the region ranging from the core surface toward the core center by less than 2 mm, or the unsaturated carboxylic acid or its salt contained in the outer core may have an average particle diameter of less than 15 μm.

The core may further contain a metal carboxylate having two or more different kinds of carboxylic acids bonded to metal, at least one of which has eight or more carbon atoms.

The difference in hardness between the core surface and the core center may be 20 or more in terms of JIS-C hardness.

The fact that cracks or breakage occur on the core surface has been found from studies on the reasons that golf balls having a core with a core surface hardness of 80 or more in terms of JIS-C hardness exhibit poor durability. This is supposedly because if the core surface of the golf ball is too hard, cracks occur due to being unable to resist deformation at the time of impact, thereby leading to breakage. Furthermore, as a result of further studies on the above reason by the present inventors, it has been found that since an unsaturated carboxylic acid or its salt as a co-crosslinking agent remains dispersedly in a base rubber, if the core surface is too hard, cracks occur only when the co-crosslinking agent having larger particles is contained in the core surface. In other words, according to the present invention, even if the core surface is hard and has a hardness of 80 or more in terms of JIS-C hardness, cracks can be avoided by excluding the unsaturated carboxylic acid or its salt having an average particle diameter of 90 μm or more from the region ranging from the core surface toward the core center by less than 2 mm, thereby enabling the provision of a golf ball with both improved durability and extended flight distance.

The above problem can be solved by using the unsaturated carboxylic acid or its salt having a smaller average particle diameter as the co-crosslinking agent added to the base rubber. However, since not only does it take time to form the unsaturated carboxylic acid or its salt having a smaller average particle diameter, but it also requires complicated manufacturing steps, the problem of increased manufacturing cost further arises due to longer time for manufacturing the core and in turn, manufacturing a golf ball. Therefore, the core of the present invention may comprise a multi-layer structure having at least a center core located in the center of the core and an outer core surrounding the center core and having a thickness of 2 mm or more. By making the average particle diameter of the unsaturated carboxylic acid or its salt contained in the center core 90 μm or more, the amount of usage of the unsaturated carboxylic acid or its salt having a smaller average particle diameter can be reduced and its manufacturing time can be shortened, thereby reducing the manufacturing cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically illustrating a golf ball according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of a golf ball according to the present invention will now be described in detail with reference to the accompanying drawing.

As shown in FIG. 1, a golf ball 1 of this embodiment mainly comprises a core 10 located in the center of the golf ball 1 and a cover 20 surrounding the core 10. Furthermore, the core 10 comprises a two-layer structure having a center core 12 located in the center of the golf ball 1 and an outer core 14 surrounding the center core 12. Plural dimples 22 are formed on the surface of the cover 20. Regarding this embodiment, the core having a two-layer structure will be explained as below. However, the present invention is not limited thereto, but may have a core having a single-layer structure or a core having a multi-layer structure having 3 or more layers. In addition, an intermediate layer may be disposed between the core 10 and the cover 20.

Regarding the core 10, both the center core 12 and the outer core 14 can be formed of a rubber composition comprising a rubber as its main component (hereinafter, a portion common to the center core 12 and to the outer core 14 is called the core 10). As the rubber serving as the main component (base rubber), a wide variety of synthetic rubber and natural rubber may be used, such as polybutadiene rubber (BR), styrene-butadiene rubber (SBR), natural rubber (NR), polyisoprene rubber (IR), polyurethane rubber (PU), butyl rubber (IIR), vinyl polybutadiene rubber (VBR), ethylene-propylene rubber (EPDM), nitrile rubber (NBR), and silicone rubber, although it is not limited thereto. As the polybutadiene rubber (BR), for example, 1,2-polybutadiene, cis-1,4-polybutadiene and the like may be used.

The core 10 may contain, as well as such a base rubber, co-crosslinking agent, and also may optionally contain, for example, crosslinking initiator, filler, age resistor, sulfur, organic sulfur compound and workability improving agent.

As the co-crosslinking agent, an unsaturated carboxylic acid or its salt is used. As the salt, metal salt is preferred. Examples of the unsaturated carboxylic acid or its metal salt include acrylic acid, methacrylic acid, maleic acid, fumaric acid, and zinc salt, magnesium salt and calcium salt of these substances. The co-crosslinking agent is in the form of a powder and has an average particle diameter of less than 90 μm in a region ranging from the surface 10 a of the core 10 toward its center by less than 2 mm so as to prevent cracks or breakage of the core. For example, as the co-crosslinking agent used for the outer core 14, the unsaturated carboxylic acid or its salt having an average particle diameter of less than 90 μm is used. On the other hand, as the co-crosslinking agent used for the center core 12, the unsaturated carboxylic acid or its salt having an average particle diameter of 90 μm or more can be used. This is because the hardness will not exceed 80 on the surface of the center core 12 in the case of the core having a multi-layer structure, although larger particles are contained in the surface of the center core 12.

The ratio of co-crosslinking agent is not particularly affected by the size of the average particle diameter, but the co-crosslinking agent may be contained in the center core 12 and the outer core 14 at the same ratio. Regarding the ratio of co-crosslinking agent per 100 parts by weight of the base rubber, for example, its lower limit is preferably approximately 5 parts by weight or more, and is more preferably approximately 10 parts by weight or more, and its upper limit is preferably approximately 70 parts by weight or less, and is more preferably approximately 50 parts by weight or less, although it is not limited thereto.

As the crosslinking initiator, it is preferred to use an organic peroxide, such as dicumyl peroxide, t-butyl peroxy benzoate, di-t-butyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethyl cyclohexane, although it is not limited thereto. Regarding the ratio of crosslinking initiator per 100 parts by weight of the base rubber, for example, its lower limit is preferably approximately 0.10 parts by weight or more, more preferably approximately 0.15 parts by weight or more, and still more preferably approximately 0.30 parts by weight or more, and its upper limit is preferably approximately 8 parts by weight or less, and more preferably approximately 6 parts by weight or less, although it is not limited thereto.

As the filler, for example, silver, gold, cobalt, chrome, copper, iron, germanium, manganese, molybdenum, nickel, lead, platinum, tin, titanium, tungsten, zinc, zirconium, barium sulfate, zinc oxide, and manganese oxide may be used, although it is not limited thereto. The filler is preferably in the form of a powder. Regarding the ratio of filler per 100 parts by weight of the base rubber, for example, its lower limit is preferably approximately 1 part by weight or more, more preferably approximately 2 parts by weight or more, and still more preferably approximately 3 parts by weight or more, and its upper limit is preferably approximately 100 parts by weight or less, more preferably approximately 80 parts by weight or less, and still more preferably approximately 70 parts by weight or less, although it is not limited thereto.

As the age resistor, commercially available products such as Nocrac NS-6 (manufactured by Ouchi Shinko Chemical Industrial Co., Ltd.) may be used, although it is not limited thereto. Regarding the ratio of age resistor per 100 parts by weight of the base rubber, its lower limit is preferably approximately 0.1 parts by weight or more, and more preferably approximately 0.15 parts by weight or more, and its upper limit is preferably approximately 1.0 parts by weight or less, and more preferably approximately 0.7 parts by weight or less, although not limited thereto.

The resilience of the core can be improved by adding the organic sulfur compound (peptizing agent). The organic sulfur compound is selected from thiophenols, thiocarboxylic acids, and the metal salts thereof. As thiophenols and thiocarboxylic acids, thiophenols such as pentachlorothiophenol, 4-t-butyl-o-thiophenol, 4-t-butyl thiophenol, 2-benzamide thiophenol, and the like, and thiocarboxylic acids such as thiobenzoic acid, and the like, are included. Further, as the metal salts thereof, zinc salt and the like is preferable. Regarding the ratio of organic sulfur compound per 100 parts by weight of the base rubber, its lower limit is preferably approximately 0.01 parts by weight or more, and more preferably approximately 0.1 parts by weight or more, and its upper limit is preferably approximately 5 parts by weight or less, and more preferably approximately 3 parts by weight or less, although it is not limited thereto.

Water may be added to the rubber composition. This can promote decomposition of the organic peroxide in the rubber composition. The water may be distilled water or tap water, but use of distilled water which is free of impurities is preferred. Regarding the ratio of water per 100 parts by weight of the base rubber, its lower limit is preferably approximately 0.1 parts by weight or more, and more preferably approximately 0.3 parts by weight or more, and its upper limit is preferably approximately 5 parts by weight or less, and more preferably approximately 4 parts by weight or less, although it is not limited thereto.

Furthermore, a metal monocarboxylate may be used instead of the water. The metal monocarboxylate introduces water into the rubber composition by way of a dehydration/condensation reaction, and thus provides an effect similar to that of water. Examples of metal monocarboxylate include Na, K, Li, Zn, Cu, Mg, Ca, Co, Ni, Pb and the like of acrylic acid, methacrylic acid, maleic acid, fumaric acid, and stearic acid, although it is not limited thereto. Of these examples, it is preferred to use Zn. Specific examples of metal monocarboxylate include zinc monoacrylate, zinc monomethacrylate, and zinc stearate, with the use of zinc monoacrylate being especially preferred. Regarding the ratio of metal monocarboxylate per 100 parts by weight of the base rubber, its lower limit is preferably approximately 1 part by weight or more, and more preferably approximately 3 parts by weight or more, and its upper limit is preferably approximately 60 parts by weight or less, and more preferably approximately 50 parts by weight or less, although it is not limited thereto.

The core 10 may contain a predetermined metal carboxylate to improve workability of the core. This metal carboxylate contains two or more different kinds of carboxylic acids bonded to metal in which at least one kind of the carboxylic acids has 8 or more carbon atoms. The “bond” mentioned here means a bond between a metal and carboxylic acid, and the number of bonds differs depending on the kind of metal. Specifically, sodium or potassium has one bonding site, zinc or calcium has two bonding sites, and iron or aluminum has three bonding sites. Since the metal carboxylate mentioned here needs to have two or more bonding sites, the kind of metal is limited to such metals. For example, in the case of zinc salt, assuming that zinc is bonded to a carboxylic acid A having 8 or more carbon atoms at one of two bonding sites, zinc needs to be bonded to any other carboxylic acid than the carboxylic acid A at the other bonding site. To distinguish from metal salt having double bonds (di-salt) such as zinc stearate in which the same carboxylic acids are bonded to metal, the prefix “mono” is used to express substance names.

Specifically, the metal carboxylate is a compound represented by Structural Formula (1) or (2) as follows.

R¹-M¹-R²  (1)

where, R¹ and R² respectively represent different carboxylic acids, at least one of R¹ and R² has 8 or more carbon atoms, and M¹ represents a divalent metal atom.

In the formula (2), R³, R⁴, and R⁵ respectively represent different carboxylic acids, at least one of R³, R⁴, and R⁵ has 8 or more carbon atoms, and M² represents a trivalent metal atom.

Specific examples include zinc monostearate monopalmitate, zinc monostearate monomyristate, zinc monostearate monolaurate, zinc monopalmitate monomyristate, zinc monopalmitate monolaurate, zinc monostearate monoacrylate, zinc monostearate monomethacrylate, zinc monostearate monomaleate, zinc monostearate monofumarate, zinc monopalmitate monoacrylate, zinc monopalmitate monomethacrylate, zinc monopalmitate monomaleate, zinc monopalmitate monofumarate, zinc monomyristate monoacrylate, zinc monomyristate monomethacrylate, zinc monomyristate monomaleate, zinc monomyristate monofumarate, zinc monolaurate monoacrylate, zinc monolaurate monomethacrylate, zinc monolaurate monomaleate, and zinc monolaurate monofumarate. Of these examples, zinc monostearate monoacrylate is preferred. Zinc stearate (that is used as metal carboxylate B in the Example hereinafter described) is not included in these examples.

These predetermined metal carboxylates can be easily obtained through reaction of a metal compound in the presence of plural carboxylic acids. Specifically, zinc monostearate monoacrylate can be obtained by dissolving stearic acid and acrylic acid in a reaction solution, and mixing and reacting zinc oxide suspended in a solvent in the resulting solution. Alternatively, zinc monostearate monoacrylate can be obtained by adding stearic acid and acrylic acid into a solution containing zinc oxide suspended in a solvent.

The ratio of the above predetermined metal carboxylate per 100 parts by weight of the base rubber is preferably approximately 0.1 to 50 parts by weight or more, and more preferably approximately 1 to 25 parts by weight or more, although not limited thereto. The mass ratio of this metal carboxylate per unsaturated carboxylic acid or its salt as the co-crosslinking agent is preferably 1 to 99% by mass, and more preferably 4 to 50% by mass. If the mass ratio does not reach the above range, sufficient effects of improving the workability may not be obtained. On the other hand, if the mass ratio exceeds the above range, the initial velocity of the core may be reduced.

The hardness of the core 10 largely differs depending on the position of the core. The hardness of the core 10 mentioned here is shown in terms of JIS-C hardness for all cases. The hardness at the surface 10 a of the core 10, i.e., the hardness Ho at the outer surface 10 a of the outer core 14, needs to have a hardness of 80 or more, and preferably have a hardness of 85 or more in terms of the spin rate and rebound performance. The upper limit of the hardness Ho is, for example, preferably approximately 95 or less in terms of the feel of the ball when hit, although it is not limited thereto.

The hardness at the center of the core 10, i.e., the hardness Hc at the center of the center core 12, is preferably 45 or more, more preferably 50 or more, and still more preferably 55 or more in terms of the durability, although it is not limited thereto. The upper limit of the hardness Hc is preferably 65 or less, more preferably 62 or less, and still more preferably 60 or less in terms of the spin rate, although it is not limited thereto.

The hardness difference ΔH between the core center hardness Hc and the core surface hardness H_(O) is preferably 20 or more, more preferably 25 or more, still more preferably 30 or more, and particularly preferably 35 or more in terms of the spin rate and distance, although it is not limited thereto. The upper limit of the hardness difference ΔH may be 45 or less, although not limited thereto. The core having a multi-layer structure may have a larger hardness difference ΔH compared to that of the core having a single-layer structure. However, even a core having a single-layer structure preferably has the same numerical range as that as described above at each position.

Regarding the amount of deformation under load of the core 10, i.e., the deformation amount when a load is applied to the core from its initial load of 98 N (10 kgf) up to 1,275 N (130 kgf), its lower limit is preferably 2.5 mm or more, and more preferably 3.0 mm or more in terms of the spin rate and feel of the ball when hit, and its upper limit is preferably 5.0 mm or less, and more preferably 4.5 mm or less in terms of the rebound performance, feel of the ball when hit, and durability.

Regarding the diameter of the core 10, its lower limit is preferably 30 mm or more, more preferably 32 mm or more, still more preferably 34 mm or more, and particularly preferably 36 mm or more, and its upper limit is preferably 40 mm or less, although not limited thereto. In the core having a two-layer structure, the lower limit of the thickness of the outer core 14 is preferably 10 mm or more, and more preferably 15 mm or more in terms of the co-crosslinking agent having a predetermined average particle diameter contained therein, and its upper limit is preferably 30 mm or less, and more preferably 25 mm or less, although it is not limited thereto. The diameter of the center core 12 is made to be within the range of the diameter of the core 10 as described above according to the preferred thickness of the outer core 14.

As a method of forming the core 10 having a two-layer structure, any known method may be adopted. For example, first, the material for the outer core 14 in a predetermined mold is subjected to a primary vulcanization (half vulcanization) to manufacture a pair of half cups in the form of hemispheric shells. Then, the center core 12 that has been prepared in advance is wrapped by the pair of half cups and in this state subjected to a secondary vulcanization (entire vulcanization). In other words, the vulcanization is conducted in two stages. Alternatively, the core may be formed by injection-molding the material for the outer core 14 around the center core 12.

As the material for forming the cover 20, ionomer resin, polyurethane thermoplastic elastomer, thermosetting polyurethane or a mixture of these substances, may be used as the main component, although it is not limited thereto. Furthermore, as well as the aforementioned main component, other thermoplastic elastomer, polyisocyanate compound, fatty acid or its derivative, basic inorganic metal compound or filler may be added to the cover 20.

Regarding the hardness of the cover 20, its lower limit is preferably 50 or more, and more preferably 55 or more, and its upper limit is preferably 75 or less, more preferably 70 or less, and still more preferably 65 or less in terms of Shore D hardness, although it is not limited thereto.

Regarding the thickness of the cover 20, its lower limit is preferably 0.2 mm or more, and more preferably 0.4 mm or more, and its upper limit is preferably 4 mm or less, more preferably 3 mm or less, and still more preferably 2 mm or less. Plural dimples 22 are formed on the surface of the cover 20. The size, shape, and quantity of the dimples 22 may be designed appropriately corresponding to a desired aerodynamic characteristic of the golf ball 1.

The cover 20 may be formed by, for example, an injection molding method, although it is not limited thereto. Specifically, the core 10 formed by the aforementioned method is placed in the center inside the mold for the cover. Then, the cover material is injected and introduced to the inside of the mold to cover the core 10. In this way, the cover 20 can be formed.

An intermediate layer (not shown) may be disposed optionally between the core 10 and the cover 20. An intermediate layer which functions as a core or a cover may be provided. Furthermore, plural intermediate layers may be provided. For example, plural intermediate layers which function as the core or the cover may be provided, or a first intermediate layer which functions as the core and a second intermediate layer which functions as the cover may be provided.

Example

Golf balls comprising a core having a composition (parts by weight as a unit) and configuration shown in Table 1 were manufactured, and then tests were carried out about various aspects of performance of the core and golf ball, especially, the productivity of the core, the spin performance of the golf ball, and the durability of the golf ball. The results are shown in Table 2.

All the golf balls in Examples and Comparative Examples contained the same materials having the same composition mainly comprising ionomer resin and had dimples arranged in the same manner.

TABLE 1 Examples Comparative examples 1 2 3 4 5 1 2 3 4 Center Butadiene rubber 100 100 100 100 100 100 100 100 100 core Zinc oxide 4 4 4 4 4 4 4 4 4 Barium sulfate 19.9 19.9 19.9 9.12 13.15 19.9 19.9 19.9 13.15 Age resister 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Pentachlorothiophenol zinc salt 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 Unsaturated metal carboxylate A — — — 35.7 28.1 — — — — Unsaturated metal carboxylate B 15 15 15 — — 15 15 15 28.1 Metal carboxylate A 2.2 2.2 2.2 6.3 — 2.2 2.2 2.2 — Metal carboxylate B — — — — 4.9 — — — 4.9 Organic peroxide A — — — 0.6 0.6 — — — 0.6 Organic peroxide B 1.8 1.8 1.8 1.2 1.2 1.8 1.8 1.8 1.2 Distilled water — — — 1 — — — — — Outer Butadiene rubber 100 100 100 — — 100 100 100 — core Zinc oxide 4 4 4 — — 4 4 4 — Barium sulfate 15.4 15.4 7.5 — — 15.4 7.5 14.1 — Age resister 0.1 0.1 0.1 — — 0.1 0.1 0.1 — Pentachlorothiophenol zinc salt 0.1 0.1 0.2 — — 0.1 0.2 0.1 — Unsaturated metal carboxylate A 23.8 23.8 39.1 — — — — — — Unsaturated metal carboxylate B — — — — — 23.8 39.1 26.4 — Metal carboxylate A 4.2 — — — — — — — — Metal carboxylate B — 4.2 6.9 — — 4.2 6.9 4.2 — Organic peroxide A 0.6 0.6 0.6 — — 0.6 0.6 0.3 — Organic peroxide B 1.2 1.2 1.2 — — 1.2 1.2 0.3 — Distilled water — — 1 — — — 1 — —

TABLE 2 Examples Comparative examples 1 2 3 4 5 1 2 3 4 Amount of deformation under load 3.58 3.60 3.57 2.85 2.86 3.60 3.57 3.60 2.84 (mm) Hardness Hardness H_(C) at core 47.8 47.8 47.8 60.5 65.6 47.8 47.8 47.8 65.9 profile center (JIS-C) Hardness H_(S) at center 65.7 65.7 65.7 89.2 86.2 65.7 65.7 65.7 86.3 core surface Hardness difference ΔH₀ 17.9 17.9 17.9 28.7 20.6 17.9 17.9 17.9 20.4 between H_(C) and H_(S) Hardness H_(I) at outer core 71.2 71 68.8 — — 71.2 68.8 72.3 — inner surface Hardness H_(O) at outer 80.5 80.3 85 — — 80.5 84.4 78 — core outer surface Hardness difference ΔH₁ 32.7 32.5 37.2 — — 32.7 36.6 30.2 — between H_(O) and H_(C) Diameter of center core (mm) 23 23 23 38.6 38.6 23 23 23 38.6 Diameter to outer core (mm) 38.6 38.6 38.6 — — 38.6 38.6 38.6 — Productivity (center core) Very Very Very Very Good Very Very Very Good good good good good good good good Productivity (outer core) Very Good Good — — Good Good Good — good Initial velocity (m/s) 77.30 77.25 77.30 77.88 77.83 77.25 77.30 77.25 77.83 Spin rate* (rpm) 0 0 −100 −100 0 0 −100 50 0 COR durability 150 150 130 120 150 90 70 150 90 (*Examples 1 to 3 and Comparative Examples 2 and 3 show a value increased or decreased with respect to the spin rate in Comparative Example 1. Examples 4 and 5 show a value increased or decreased with respect to the spin rate in Comparative Example 4.)

As polybutadiene rubber in Table 1, “BR730”, which is the trade name, available from JSR Corporation, was used for the base rubber.

As zinc oxide, “Zinc Oxide Grade 3”, which is the trade name, available from Sakai Chemical Co., Ltd., was used.

As barium sulfate, “Barico #100”, which is the trade name, available from Hakusuitech Co., Ltd., was used.

As age resistor, “Nocrac NS-6”, which is the trade name, available from Ouchi Shinko Chemical Industrial Co., Ltd., was used.

As pentachlorothiophenol zinc salt, one available from Wako Pure Chemical Industries, Ltd., was used.

As unsaturated metal carboxylate A, zinc acrylate having an average particle diameter of 7 to 15 μm, available from Wako Pure Chemical Industries, Ltd., was used.

As unsaturated metal carboxylate B, zinc acrylate having an average particle diameter of 90 to 150 μm, available from Wako Pure Chemical Industries, Ltd., was used.

The average particle diameter represents a value obtained from measuring particle size distribution by a sedimentation method using“CAPA-700”, manufactured by Horiba, Ltd. The range of the average particle diameter was between the maximum value and the minimum value of the measured values of 5 measurement results.

As metal carboxylate A, zinc monostearate monoacrylate, available from Nippon Shokubai Co., Ltd., was used.

As metal carboxylate B, zinc stearate, available from Wako Pure Chemical Industries, Ltd., was used.

As organic peroxide A, “Percumyl D”, which is the trade name, available from NOF Corporation, was used as crosslinking initiator.

As organic peroxide B, “Perhexa C-40”, which is the trade name, available from NOF Corporation, was used as crosslinking initiator.

The amount of deformation under load in Table 2 is a deformation amount (mm) when applying a load of 100 kg to the core. It is shown that the greater the deformation amount is, the softer the core is.

Regarding the core hardness profile in Table 2, its measuring method will be explained. To obtain the hardness Hc at the core center, measurement was carried out by applying the indenter of a durometer to the center of the cross-section obtained by cutting a spherical core in half through the center. To obtain the hardness H_(S) at the center core surface, the hardness H_(I) at the outer core inner surface, and the hardness H_(O) at the outer core outer surface, measurements were carried out by pressing the indenter of the durometer perpendicularly against these surfaces to be measured. As the durometer, a JIS-C spring-type durometer, as specified in JIS K 6301-1975, was used. The above hardness is a measured value obtained after holding the core isothermally at 23° C.

Regarding the productivity in Table 2, when kneading and extruding the core composition, (i) kneading time, (ii) adhesion to the inner wall of a kneading device (residue), (iii) integration of the core compositions after kneading, and (iv) core surface roughness when being extruded were mainly evaluated. After these are comprehensively judged, the result was evaluated as “Very good” when the core productivity was extremely high, “Good” when it was high, and “Poor” when it was low.

Regarding the initial velocity in Table 2, it was measured by using the initial velocity measuring instrument which is the same system as the drum rotary initial velocity meter of United States Golf Association (USGA) which is the device approved by Royal and Ancient Golf Club of Saint Andrews (R&A). The core and golf ball were tested in a chamber at a room temperature of 23±2° C. after being held isothermally in a 23.9±1° C. environment for at least 3 hours. The ball was hit using a 250-pound (113.4 kg) head at an impact velocity of 143.8 ft/s (43.83 m/s). Thirty balls of each sample were each hit twice. The time taken for the ball to traverse a distance of 6.28 ft (1.91 m) was measured and used to compute the initial velocity (m/s).

Regarding the COR durability in Table 2, the durability of the golf balls was evaluated by using ADC Ball COR Durability Tester of Automated Design Corporation (U.S.). The tester has a function for ejecting golf balls by air pressure and colliding them consecutively on two metal plates installed in parallel. The velocity of striking to the metal plates was 43 m/s. An average value of the number of times of ejections of the golf ball given until the ball was broken was determined. In these test results, the average value refers to a value obtained by averaging the number of times of ejections given until all ten balls ejected for each Example and each Comparative Example were broken. The numeric value was round off to the nearest ten.

Regarding the spin rate in Table 2, with a driver (Tour Stage X-Drive Type 455 9.5° manufactured by Bridgestone Sports Co., Ltd.) mounted on a swing robot (manufactured by Miyama Co., Ltd.), a golf ball was hit at a head speed of 45 m/s, and the ball just after it was hit was photographed with a high-speed camera to measure its spin rate (rpm).

In Examples 1 to 3 relating to a core having a two-layer structure and having a core surface hardness H_(O) of 80 or more, an unsaturated metal carboxylate having an average particle diameter of 7 to 15 μm was used as the co-crosslinking agent for the outer core. Thus, as shown in Tables 1 and 2, even if an unsaturated metal carboxylate having an average particle diameter of 90 to 150 μm was used as the co-crosslinking agent for the center core, the result showed an excellent durability while cracks or splits hardly occurred.

In Comparative Examples 1 and 2 relating to a core having a two-layer structure and having a core surface hardness H_(O) of 80 or more, an unsaturated metal carboxylate having an average particle diameter of 90 to 150 μm was used as the co-crosslinking agent for both the center core and the outer core. Thus, the result showed a remarkably poor durability compared to the results in Examples 1 to 3. On investigating the golf balls after being subjected to the durability test, it was observed that cracks occurred in a region about 1 mm inner from the core surface. As a result of this, it was found that a material having a larger particle diameter cannot be used, broadly, in a region about 2 mm inner from the core surface so as to prevent cracks from occurring.

In Comparative Example 3 relating to a core having a two-layer structure, an unsaturated metal carboxylate having an average particle diameter of 90 to 150 μm was used as the co-crosslinking agent for both the center core and the outer core. The result showed durability similar to that of Examples 1 to 3, since the core had a surface hardness H_(O) of 78, which is low. However, since the core had a low surface hardness, the hardness difference between the core surface and the core center was small. Therefore, since the spin rate in Comparative Example 3 increased compared to that in Examples 1 and 2 and Comparative Example 1, sufficient flight distance was not obtained. Regarding the flight distance, in Example 3 in which the core has a surface hardness Ho of 85 or more and the hardness difference ΔH₁ between the core surface and the core center is 35 or more, the spin rate was decreased compared to that in Examples 1 and 2 and the flight distance was further improved.

In Examples 4 and 5 relating to a core having a single-layer structure and having a center core surface hardness Hs of 85 or more, an unsaturated metal carboxylate having an average particle diameter of 7 to 15 μm was used as the co-crosslinking agent for the center core. Thus, the result showed excellent durability while cracks or splits hardly occurred. On the other hand, in Comparative Example 4 relating to a core having a single-layer structure and having a core surface hardness Ho of 85 or more, an unsaturated metal carboxylate having an average particle diameter of 90 to 150 μm was used as the co-crosslinking agent for the center core. Thus, the result showed a remarkably poor durability compared to the result in Examples 4 and 5. On examining the golf balls after being subjected to the durability test, it was also observed that cracks occurred in a region about 1 mm inward from the core surface. 

What is claimed is:
 1. A golf ball comprising a core that comprises a base rubber, and an unsaturated carboxylic acid or its salt as a co-crosslinking agent, wherein the core has a surface hardness of 80 or more in terms of JIS-C hardness, and wherein the unsaturated carboxylic acid or its salt contained in a region ranging from a surface of the core toward its center by less than 2 mm has an average particle diameter of less than 90 μm.
 2. The golf ball according to claim 1, wherein the core comprises a multi-layer structure having at least a center core located in the center of the core and an outer core that surrounds the center core and has a thickness of 2 mm or more, and wherein the unsaturated carboxylic acid or its salt contained in the center core has an average particle diameter of 90 μm or more, and the unsaturated carboxylic acid or its salt contained in the outer core has an average particle diameter of less than 90 μm.
 3. The golf ball according to claim 1, wherein the unsaturated carboxylic acid or its salt contained in the region ranging from the surface of the core toward its center by less than 2 mm, or the unsaturated carboxylic acid or its salt contained in the outer core has an average particle diameter of less than 15 μm.
 4. The golf ball according to claim 1, wherein the core further includes a metal carboxylate that contains two or more different kinds of carboxylic acids bonded to metal in which at least one kind of the carboxylic acids has 8 or more carbon atoms.
 5. The golf ball according to claim 1, wherein the difference in hardness between the surface of the core and the center of the core is 20 or more in terms of JIS-C hardness. 